Automotive vehicles with an internal combustion engine have an exhaust system including a pathway for exhaust gas to move away from the engine. Depending on the desired operating state, internal combustion engines can be operated with fuel/air ratios in which (1) the fuel constituent is present in a stoichiometric surplus (rich range), (2) the oxygen of the air constituent is stoichiometrically predominant (lean range), and (3) the fuel and air constituents satisfy stoichiometric requirements. The composition of the fuel-air mixture determines the composition of the exhaust gas. In the rich range, considerable quantities of nonburned or partially burned fuel are found, while the oxygen has been substantially consumed and has nearly disappeared. In the lean range, the ratios are reversed, and in a stoichiometric composition of the fuel-air mixture, both fuel and oxygen are minimized.
It is well known that the oxygen concentration in the exhaust gas of an engine has a direct relationship to the air-to-fuel ratio of the fuel mixture supplied to the engine. As a result, gas sensors, namely oxygen sensors, are used in automotive internal combustion control systems to provide accurate oxygen concentration measurements of automobile exhaust gases. They are used for determination of optimum combustion conditions, maximization of fuel economy, and management of exhaust emissions.
A switch type oxygen sensor, generally, comprises an ionically conductive solid electrolyte material, a sensing electrode that is exposed to the exhaust gas, and reference electrode that is exposed to a reference gas. Reference gases including air or oxygen, at known partial pressures are used. The sensor operates in potentiometric mode, where oxygen partial pressure differences between the exhaust gas and reference gas on opposing faces of the electrochemical cell develop an electromotive force, which can be described by the Nernst equation:   E  =            (              RT                  4          ⁢          F                    )        ⁢          xe2x80x83        ⁢    ln    ⁢          xe2x80x83        ⁢          (                        P                      O            2                    ref                          P                      O            2                              )      
where:
E=electromotive force
R=universal gas constant
F=Faraday constant
T=absolute temperature of the gas
PO2ref=oxygen partial pressure of the reference gas
PO2=oxygen partial pressure of the exhaust gas
The presence of a large oxygen partial pressure difference between rich and lean exhaust gas conditions creates a step-like difference in cell output at the stoichiometric point; the switch-like behavior of the sensor enables engine combustion control about stoichiometry. Stoichiometric exhaust gas, which contains unburned hydrocarbons, carbon monoxide, and oxides of nitrogen is a favored condition because these materials can be converted very efficiently to water, carbon dioxide, and nitrogen by automotive three-way catalysts in automotive catalytic converters. Also, in addition to their value for emissions control, the sensors provide improved fuel economy and drivability.
Further control of engine combustion can be obtained using amperometric mode exhaust sensors, where oxygen is electrochemically pumped through an electrochemical cell using an applied voltage. A gas diffusion-limiting barrier may be used to create a current limited output, the level of which is proportional to the oxygen content of the exhaust gas. These sensors typically consist of two or more electrochemical cells; one of these cells operates in potentiometric mode and serves as a reference cell, while another operates in amperometric mode and serves as an oxygen-pumping cell. This type of sensor, known as a wide range or linear air/fuel ratio sensor, provides information beyond whether the exhaust gas is qualitatively rich or lean; it can quantitatively measure the air/fuel ratio of the exhaust gas.
Due to increasing demands for improved fuel utilization and emissions control, more recent emphasis has been on wide range oxygen sensors capable of accurately determining the oxygen partial pressure in exhaust gas for internal combustion engines operating under both fuel-rich and fuel-lean conditions. Such conditions require an oxygen sensor that is capable of rapid response to changes in oxygen partial pressure by several orders of magnitude, while also having sufficient sensitivity to accurately determine the oxygen partial pressure in both the fuel-rich and fuel-lean conditions. One way to obtain such sensors is by providing temperature compensation to the sensor.
The temperature of the exhaust gases ranges from ambient temperature, when the engine has not been run recently, to higher than 1,000xc2x0 C. Since air-fuel ratio output signal depends largely on the exhaust gas temperature, temperature compensation is needed. A heater assists an oxygen sensor in making more precise measurements over a wide range of exhaust gas temperatures, especially when the exhaust gas temperature is low. The addition of the heater also helps to decrease the light-off time of the sensor, that is, the time that it takes for the sensor to reach the minimum temperature for proper operation.
Reduction of light-off times has been accomplished through the use of high power heaters. One method for further decreasing light-off times while using only small or modest heating power, is to substantially decrease the size of the sensing element, especially the electrolyte. Similarly, during low temperature operation (e.g., about 350xc2x0 C. or less), the switching time, or time required for the sensor to detect a change from rich to lean or lean to rich exhaust gas compositions, must be as low as possible, preferably below about half a second (500 milliseconds).
The surface geometry and the availability of the electrode to the exhaust gas is a factor that affects the sensitivity and response time of an exhaust gas sensor. Also, the thinner the electrolyte, and the more porous the electrode, the more rapid and more sensitive is the sensor. Planer sensors offer a benefit of large surface area, while affording the possibility of a relatively thin electrolyte. A method of making gas sensor that would allow for accurate determination of the oxygen content in an exhaust gas would be useful.
Disclosed herein is a method of making a gas sensor, comprising disposing a reference electrode on an inner surface of an electrolyte, sputtering a sensing electrode on an outer surface of the electrolyte, sputtering a zirconia layer on a side of the sensing electrode opposite the electrolyte, wherein the zirconia layer has a thickness of about 20 nm to about 1,000 nm, and disposing a protective layer on a side of the zirconia layer opposite the sensing electrode.
Also disclosed is a gas sensor, comprising a reference electrode deposited on an inner surface of an electrolyte, a sensing electrode sputtered on an outer surface of the electrolyte, a zirconia layer sputtered on a side of the sensing electrode opposite the electrolyte, wherein the zirconia layer has a thickness of about 20 nanometers to about 1,000 nm; and a protective layer deposited on a side of the zirconia layer opposite the sensing electrode. These and other features will be apparent from the following brief description of the drawings, detailed description, and attached drawings.
In addition, disclosed is a method of making a gas sensor, comprising disposing a reference electrode on an inner surface of an electrolyte, sputtering a sensing electrode comprising platinum and aluminum, optionally comprising ruthenium, rhodium or a combination comprising one of the foregoing on an outer surface of the electrolyte, wherein the sputtering comprises xenon, neon, or a combination comprising one of the foregoing, and wherein the temperature of the electrolyte is less than or equal to about xe2x88x9280xc2x0 C. during at least a portion of the sputtering, xenon ion cleaning the sensing electrode, etching the sensing electrode with an etching solution having a pH greater than 7, heat-treating at a temperature of about 400xc2x0 C. to about 1200xc2x0 C., sputtering a zirconia layer on a side of the sensing electrode opposite the electrolyte, wherein the zirconia layer has a porosity of about 2 vol % to about 8 vol %, based on the total volume of the zirconia layer, and disposing a protective layer on a side of the zirconia layer opposite the sensing electrode, wherein the protective layer has a porosity of about 30 vol % to about 60 vol %, based on the total volume of the protective layer.